This invention relates to ultrasonic surgical instruments and associated methods of use. More particularly, this invention relates to the treatment of wounds with ultrasound energy. The treatment contemplated by this invention includes fragmentation and emulsification of hard and soft tissue in a clinical environment while reducing unwanted heat and collateral tissue damage.
Over the past 30 years, several ultrasonic tools have been invented which can be used to ablate or cut tissue in surgery. Such devices are disclosed by Wuchinich et al. in U.S. Pat. No. 4,223,676 and Idemoto et al in U.S. Pat. No. 5,188,102.
In practice, these surgical devices include a blunt tip hollow probe that vibrates at frequencies between 20 kc and 100 kc, with amplitudes up to 300 microns or more. Such devices ablate tissue by either producing cavitation bubbles which implode and disrupt cells, by generating tissue compression and relaxation stresses (sometimes called the jackhammer effect) or by other mechanisms such as micro streaming of bubbles in the tissue matrix. The effect is that the tissue becomes liquefied and separated. The fragmented tissue becomes emulsified with an irrigant solution. The resulting emulsion or slurry of tissue debris is then aspirated from the site. Bulk excision of tissue is possible by applying the energy around and under an unwanted tissue mass to separate it from the surrounding structure. The surgeon can then lift the separated tissue mass out using common tools such as forceps.
The tubular probe is excited by a transducer of either the piezoelectric or magnetostrictive type that transforms an alternating electrical signal within the frequencies indicated above into a longitudinal or transverse vibration. When the probe is attached to the transducer, the two become a single element with series and parallel resonances. The designer will try to tailor the mechanical and electrical characteristics of these elements to provide the proper frequency of operation. Most of the time, the elements will have a long axis that is straight and has the tip truncated in a plane perpendicular to the long axis. This is done for simplicity and economic considerations. In almost all applications, whether medical or industrial, such an embodiment is practical and useful. However, in applications such as the debridement of burns, wounds, diabetic ulcers or ulcers induced by radiation treatments, the blunt straight probe has been shown to be less effective in removing the hard eschar buildup that occurs when the wound is healing. This eschar buildup must be removed so that the healthy tissue is exposed and allowed to close the wound to provide complete healing with minimal scar tissue formation. Also, the small diameter tip, since it is cannulated, has a small annular area with limits energy transmission into the wound. This extends the length of the procedure and causes operator fatigue and patient discomfort.
Therefore, it was desired to provide a probe that can be mated to an ultrasonic surgical aspirator that increases the efficiency of emulsification, does not heat up the operative site and lowers the time of operation.
In response to this need, a series of devices were developed which have been proven to address at least some of the shortcomings of the previous techniques. These devices are described in U.S. Pat. No. 7,931,611 issued Apr. 26, 2011. The devices have been shown to be effective in clinical use for the removal of necrotic tissue and some softer types of eschar.
High temperature burns produce a tough, leathery type of eschar. While the eschar can slough off naturally, its removal through surgical debridement is often necessary in order to prevent infection. This is even more important for immuno-compromised patients. A frequently used eschar removal option involves the use of a manual dermatome such as a Weck knife. The surgeon cuts thin slices of eschar until the healthy tissue is exposed. Bleeding is the key visual indicator when the Weck knife has reached healthy tissue. Ideally, the eschar should be removed without any insult to the healthy tissue below. As this is not possible, the thinner the layer of healthy tissue removed during debridement, the more successful the eschar removal procedure.
The ultrasonic debridement of wounds, as described above, has already been proven as an extremely precise, necrotic tissue removal method with the added benefit of minimum impact to the healthy tissue. Ultrasound wound debridement probes are used for debriding complex tissue topographies with minimal loss of healthy tissue. This is not possible to duplicate with sharps, such as scalpels or Weck knives.
Because some of the mechanical properties of the high temperature-induced eschar, such as elasticity, are close to those of healthy tissue, ultrasonic debridement using the ultrasonic debridement tools developed to date is problematic.